Holographic correlators offer the possibility of correlating two-dimensional images or data against multiple reference images in a very short time. The reference images are recorded in a holographic recording medium, each image having a respective recording angle and hence a respective diffraction angle when the recorded medium is used as a filter. Then, an unknown image is impressed in a collimated beam incident upon the filter. The intensity of the light diffracted by the filter at a respective diffraction angle represents the correlation between the unknown image and the reference image associated with that diffraction angle. If the unknown image corresponds exactly to one of the reference images, a single optical beam is produced. If the unknown image is "close" to one of the reference images but significant differences exist, the diffraction angle corresponding to the "close" reference image will have a high intensity, but the remaining diffraction angles will likely have finite but smaller intensities. Thus, the reference image that produces the highest intensity output for an unknown image represents the best choice for that unknown image. Such a correlator has obvious application for pattern recognition, image processing, and neural networks.
The VanderLugt correlator has been widely used for a holographic correlator. However, the systems reported to date have been bulky and extremely sensitive to the positioning of the filter.
An incoherent holographic correlator overcomes some of these problems. It requires a wide light beam that is temporally coherent and spatially incoherent. Temporal coherency is required to faithfully reconstruct without dispersion the image stored in the holographic filter. Spatial incoherence is required to average out the interference terms which are sensitive to the phase difference between different parts of the unknown image. One light source used in prior-art incoherent correlators has been a widely collimated standard laser beam passing through a rotating diffuser. Another light source has been a cathode ray tube whose screen is covered with a spectral filter having a narrow linewidth. Neither solution has been fully satisfactory. The diffuser or narrow bandwidth filter reduces the light intensity. The physical rotation of the diffuser presents mechanical problems. The CRT is bulky.
The present inventor has disclosed a holographic memory in U.S. Pat. No. 4,988,153. This memory holographically records images in a medium and selects an image by activating one of many lasers integrated in a two-dimensional array. He has also disclosed a holographic learning machine in U.S. patent application Ser. No. 07/588,710, filed Sep. 27, 1990 and together with others in "Holographic On-Line Learning machine for Multicategory Classification," Japanese Journal of Applied Physics, volume 19, 1990, pp. L1332-L1334. In the learning machine, a linear array of mutually coherent intensity-modulated laser beams provide reference beams simultaneously irradiating a recording medium in order to correct the diffraction patterns produced by a spatially coherent teaching image simultaneously irradiating the medium.